1. Technical Field
The present invention relates to rotors for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators, and to methods of manufacturing the rotors. In addition, the invention can also be applied to industrial machines and household electrical appliances.
2. Description of Related Art
There are known rotors for electric rotating machines which include a rotor core formed by stacking a plurality of steel sheet blocks and a rotating component (or rotating object) configured to rotate together with the rotor core.
Here, when the electric rotating machine is of an inner rotor type, the rotating component represents a rotating shaft that is disposed radially inside of the rotor core so as to extend in the axial direction of the rotor core; otherwise, when the electric rotating machine is of an outer rotor type, the rotating component represents a rotating drum that is fitted on the radially outer periphery of the rotor core. In addition, each of the steel sheet blocks is obtained by laminating a plurality of magnetic steel sheets.
Further, each of the steel sheet blocks has a positioning portion for positioning the rotor core with respect to the rotating component in the circumferential direction of the rotor core.
For example, as shown in FIGS. 10A and 10B, each of the steel sheet blocks 101 may have a key 103 as the positioning portion. More specifically, the key 103 is formed so as to protrude radially inward from the inner circumferential surface (or the radially inner surface) of the steel sheet block 101 which has an annular shape. The key 103 is fitted into a keyway 105 formed in the radially outer surface of the rotating shaft 102 (i.e., the rotating component), thereby positioning the steel sheet block 101 with respect to the rotating shaft 102 in the circumferential direction.
Furthermore, the magnetic steel sheets, of which the steel sheet blocks 101 are formed, are generally shaped by punching a rolled magnetic steel sheet. In this case, the magnetic characteristics of each of the steel sheet blocks 101 vary in the circumferential direction depending on the direction of rolling of the magnetic steel sheets that make up the steel sheet block 101, thereby causing magnetic imbalance in the rotor core.
Moreover, there is also known a technique for reducing the magnetic imbalance in the rotor core. According to the technique, the steel sheet blocks 101 are first rotated by a predetermined angle with respect to each other and then stacked together. Consequently, when viewed along the axial direction of the resultant rotor core, the directions of rolling of the magnetic steel sheets that make up the steel sheet blocks 101 intersect each other.
With the above technique, it is possible to reduce the magnetic imbalance in the rotor core without employing more than one type of the magnetic steel sheets. However, at the same time, the above technique also involves the following problem.
For each of the steel sheet blocks 101, the direction of rolling of the magnetic steel sheets that make up the steel sheet block 101 is represented by an imaginary line X that extends straight, on a plane parallel to the steel sheet block 101, through the center of rotation of the rotor core so as to circumferentially bisect the key 103 of the steel sheet block 101. Therefore, when the steel sheet blocks 101 are rotated with respect to each other so as to make the directions of rolling of the magnetic steel sheets that make up the steel sheet blocks 101 circumferentially offset from each other, the circumferential positions of the keys 103 of the steel sheet blocks 101 accordingly become different from each other. Consequently, it is necessary to form, in the radially outer surface of the rotating shaft 102, a plurality of keyways 105 that are circumferentially offset from each other as shown in FIG. 10B.
On the other hand, Japanese Patent No. 4070674 discloses a technique for achieving a skewed rotor structure by employing only a single type of steel sheets. More specifically, according to the technique, a rotor core is comprised of a plurality of identical steel sheets; the steel sheets are laminated with part of the steel sheets front-back inverted with respect to and thus being a mirror image of the other steel sheets. However, this patent document fails to disclose or even suggest how to reduce magnetic imbalance that is caused in the rotor core depending on the directions of rolling of the steel sheets.